This invention pertains to tracing flow of fluids from a first point to a second point in an underground reservoir by injecting a small amount of a tracer material into the fluid at the first point. More particularly, this invention pertains to tracing the flow of a fluid, such as oil for example, from one point to a second point in an underground reservoir by injecting into the fluid at the first point one or more stable, free radicals, or spin labels, which are detectable by electron spin resonance analysis (ESR) to provide both quantitative and qualitative determination of the spin labels in a sample of the fluid that has been removed from the reservoir at the second point.
Both water and petroleum are produced from underground reservoirs wherein they reside in a liquid state. Efficient and economic production of such fluids is dependent upon the flow pattern thereof to a producing well site. Problems such as unexpected anisotropic permeability, unsuspected fracture orientation, uncertainty of communication between horizons, unknown stratigraphic and structural barriers, and so forth, plague the reservoir engineer faced with the task of predetermining all obstacles to efficient and economical reservoir management, yet responsible for operating a profitable fluid recovery operation. The reservoir engineer must depend, therefore, upon prompt detection and identification of problems when they do occur in order to apply remedial measures.
The need for an effective tracer system for determining flow patterns in a fluid reservoir, and the problems associated with the development of such a system, is described by Troutman and Schutz in their paper "Field Applications of Radioactive Tracers in Secondary Recovery" which appeared in Europe and Oil, June, 1970. They stated that a frequent manifestation of fluid flow problems in a reservoir is the appearance of an injected fluid at a producing well (second point) at a time other than that predicted. When this situation occurs, it is essential to determine the source of the injected fluid being produced, i.e. from which injection well (first point) did it come. If a different tracer chemical is added to each of several injection wells, the presence of these tracers in produced fluids will uniquely and immediately identify the injection site. In a secondary recovery process for oil, for instance, the source well of a drive fluid, such as injected water, can be determined. Accordingly, the flow pattern of oil and drive fluid to a producing well can be mapped.
To be a practical tracer, the material employed must be safe to handle and reasonable in cost. It must behave, in time of travel from the injection point to the producing point, as the fluid being traced and it must be detectable in the produced fluid, preferrable both qualitatively and quantitatively.
The use of tracer materials for tracing fluid flow in underground reservoirs is not new. Numerous materials which have been tried as tracers include various dyes; gases such as helium and carbon dioxide; acids such as picric acid, salicylic acid, ethylenediaminetetraacetic acid (EDTA), or the salts thereof; ionizable compounds which provide ammonium, boron (as borate), bromide, chromate, dichromate, iodide, nitrate or thiocynate ions; formaldehyde; carbon disulfide; and radioactive materials such as tritiated water (HTO), tritiated hydrogen (HT), tritated methane, and krypton-85.
With few exceptions, all of the previously mentioned tracer compounds have not proven to be altogether satisfactory as tracers. Other than for certain special situations, the dyes, nonradioisotopic materials, and gases are rarely used today, either because of cost considerations and/or the poor detectability thereof in the produced fluid being traced. Being detectable and measurable at concentrations as low as about 2 .times. 10.sup.-.sup.16 gram-mole, the use of radioisotopes has become more accepted and is still employed, but radioactive materials have the disadvantages of being relatively expensive, in having to match the tracer type used to the fluid in the formation, in requiring specialized training and a license to assure their safe handling, and in requiring a thorough knowledge of, and availability of, sophisticated detection techniques and apparatus.
Therefore, a principle object of the present invention is to provide a tracer method for tracing fluid flow in an underground reservoir whereby the aforementioned problems associated with previous tracer methods are avoided.
One specific object of the present invention is to provide a tracer method which relies on a tracer material that is relatively low in cost, is safe, and is detectable and measurable at concentrations far lower than previously employed nonradioisotopic chemical tracers, yet which is detectable and measurable at low concentrations approaching those at which radioistropic tracers are detectable and measurable.
Another object of the present invention is to provide a tracer method which is dependent on use of tracer chemicals which are not found in nature.